The present invention relates to a semiconductor optical modulator of an optical waveguide type which uses a glass fiber as a transmission medium and is used for optical communication in a wavelength range of 1.3 to 1.5 μm and a laser with an optical modulator.
With the recent development of an advanced information-oriented society, optical communication networks using optical fibers have developed in various countries of the world, and many communications are made by optical communication for telephones and data transmission.
In order to realize optical communication using an optical fiber, an electric signal must be converted into an optical signal by using a transducer. A device for such electrooptic conversion is an optical modulator.
Optical modulation for electrooptic conversion includes modulators using two schemes, i.e., a direct modulation scheme and an external modulation scheme. In the direct modulation scheme, a semiconductor laser diode (LD) is used to directly convert a change in a modulation signal into a change in the intensity of a light source. In contrast to this, in the external modulation scheme, output light from a semiconductor laser under the continuous wave (CW) operation is externally modulated.
Direction modulation using a semiconductor laser allows a simple arrangement and a reduction in size. For this reason, direct modulation using a semiconductor laser has been widely used. In this modulation scheme, however, the transmission rate is limited by the chirping phenomenon of a semiconductor laser with a high frequency as high as several GHz or more. Chirping is a wavelength fluctuation phenomenon in which the refractive index of an active layer varies with temporal variations in carrier under high-speed modulation (several GHz or more) of the semiconductor laser, and the wavelength of emitted light varies.
In contrast to this, an external modulator modulates stable light from a semiconductor laser under CW operation by an electrooptic effect and the like, and hence is free from the problem of chirping. Therefore, this modulator allows long haul and high bit-rate transmission.
In optical communication, signal light in a wavelength range of 1.3 to 1.5 μm is used, and a silica fiber is mainly used as a transmission medium. In such optical communication, a semiconductor optical modulator made of a compound semiconductor (crystal) such as InGaAlAs or InGaAsP is used (see patent reference 1).
Recently, with advances in techniques of forming very thin compound semiconductor films, e.g., molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOVPE), semiconductor multiple-quantum-well (MQW) structures and superlattice structures have made their appearances. These structures make it possible to make considerable improvements in the characteristics of optical devices, e.g., improvements in modulation speed and so on, as compared with conventional bulk semiconductors. Of these improvements, improvements in electroabsorption effect obtained by using an MQW structure for a light absorption layer and changing absorption coefficients by applying electric fields to the MQW structure are especially notable compared with bulk semiconductors.
As the above semiconductor optical modulator, a device that can be driven at higher speeds and lower voltages has been realized by using these characteristics. This semiconductor optical modulator is designed to apply a reverse-bias electric signal to a light absorption layer having an MQW structure, and to shift the absorption edge wavelength of an exciton, thereby modulating input CW light (light source) and converting an electric signal into an optical signal.
[Patent Reference 1]
Japanese Patent Laid-Open No. 8-86987
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Fabio Bernardini, Vincenzo Fiorentini, and David Vanderbilt, “Spontaneous polarization and piezoelectric constants of III-V nitrides”, Phys. Rev. B, 56 (1997) R10024
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K. Osamura, K. Nakajima and Y. Murakami, “Fundamental absorption edge GaN, InN and their alloys”, Solid State Comm., 11(1972)617
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N. Puychevrier and M. Menoret, “Synthesis of III-V semiconductor nitrides by reactive cathodic sputtering”, Thin Solid Films, 36(1976)141
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T. Matsuoka, H. Tanaka, T. Sasaki and A. Katusi, “Wide-Gap Semiconductor (In, Ga) N”, International Symposium on GaAs and Related Compounds, (Karuizawa, Japan, 1989); in Inst. Phys. Conf. Ser., 106. pp. 141–146
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T. Matsuoka, “Phase Separation in Wurtzite In1-X-YGaXAlYN”, MRS Internet J. Nitride Semicond. Res. 3,54(1998)
The conventional semiconductor optical modulators, however, have the following problems.
First of all, in a device using an InGaAsP-based material, the band discontinuity at the conduction band in a band structure is small. This device is therefore susceptible to the influences of operating temperatures. In the existing circumstances, therefore, a device using an InGaAsP-based material having a zincblende crystal structure is undesirably low in characteristic temperature, which is one of the important characteristics in practice. In an InGaAsP-based material, since there is no energy gap, i.e., spin split-off energy Δsp, between the heavy hole and the light hole at the Γ point in the valence band, the Auger effect is large. As a consequence, a device using an InGaAsP-based material has poor temperature characteristics.
In the above conventional semiconductor optical modulators, it is required to increase the shift amount of a wavelength. In order to increase the shift amount, however, a high voltage must be applied as the above reverse bias electric signal. This is because, in the existing circumstances, it is almost impossible to make a light absorption layer have an ideal state without any carrier. For example, impurities may diffuse into the light absorption layer from the p-type and n-type semiconductor layers formed on the upper or lower surfaces of the light absorption layer. As described above, in practice, carriers exist in the light absorption layer, and hence the light absorption layer cannot be completely depleted by the built-in voltage generated by a p-n junction. As a consequence, an electric signal applied to the light absorption layer is partially consumed for the depletion of the light absorption layer. The portion consumed for the depletion is required as a reverse bias electric signal.
A conventional semiconductor optical modulator is therefore required to resist a high reverse bias electric current. It is, however, not easy to grow a high-purity crystal having a high reverse breakdown voltage.
In addition, since an electronic circuit for operating an optical modulator cannot generate a high voltage under ultrafast operation, the operating voltage of the optical modulator must be minimized. For this purpose, a metal electrode formed on a semiconductor optical modulator needs to be in the state of ohmic contact between itself and a compound semiconductor layer of the semiconductor optical modulator. This indicates that the semiconductor layer with which the metal electrode is in contact has a high impurity concentration, and an active region formed from the above high-purity crystal is sandwiched by this layer with a high impurity concentration. In an external modulator for optical communication, of the materials which can be selected for layers sandwiching an MQW structure, InP has the maximum band-gap energy before the present invention. This band-gap energy is 1.42 eV. In this arrangement, when a high reverse bias is applied to increase the modulation degree, the layer around the quantum-well may be also subjected to dielectric breakdown.
An expensive crystal growth apparatus is required to manufacture such a structure in which a high-purity crystal layer and a crystal layer having a high impurity concentration are stacked on each other. In addition, a complicated process is required to grow crystal layers. That is, it is not easy to manufacture such a structure. Furthermore, many steps are required for a device manufacturing process following crystal growth. As a consequence, the above conventional semiconductor optical modulator requires a high cost to manufacture devices. Moreover, the manufacturing yield of devices is low, resulting in expensive devices.
Conventionally, a DC reverse bias and high-frequency electric signal must be simultaneously applied to a semiconductor optical modulator. For this reason, an electronic circuit component called a bias T must be used. When, for example, a semiconductor optical modulator is to be used at a communication rate of 10 Gbits/sec, the output band width required for the above electronic circuit component is DC to 60 GHz, resulting in high cost. In addition, it is difficult to obtain an electronic circuit component exhibiting good characteristics in an output range of DC.
The general arrangement and operation of a conventional optical modulator will be described in more detail below.
A conventional optical modulator has a crystal structure in which a lower cladding layer, undoped multiple-quantum-well layer, upper cladding layer, and p-type contact layer are sequentially stacked on each other. The MQW structure functions as an absorption layer which absorbs light, and constitutes an optical waveguide, together with the upper and lower cladding layers sandwiching the absorption layer. In addition, a p-electrode is formed on the contact layer, and an n-electrode is formed on the lower surface of the substrate.
Light incident through a light incident end face at one end of the optical waveguide is ON/OFF-modulated while propagating along the optical waveguide, and emerges from a light exist end face at the opposite end to the light incident end face. FIG. 14 shows the dependence of light absorption characteristics on a wavelength λ at the time (OFF) when no voltage is applied between the p-electrode and the n-electrode and at the time (ON) when a reverse bias is applied between them. Referring to FIG. 14, the wavelength λ of light incident on this optical modulator is shown as an operating wavelength λs. When no voltage is applied, the light absorption coefficient at the operating wavelength λs is sufficiently small, and hence the incident light emerges from the light exit end face without any change. In contrast to this, when a sufficient reverse bias is applied, the absorption characteristic curve of the light absorption layer moves to the long-wavelength side, and hence the light absorption increases. As a result, no light emerges from the exit end. When the reverse bias voltage is turned on/off in this manner, light propagating along the optical waveguide is turned off/on, thereby transducing an electric signal into an optical signal.
A semiconductor layer structure that forms an optical modulator is formed by crystal growth. In this growth process, p-type and n-type dopants in the cladding layers on the upper and lower surfaces of the light absorption layer are diffused during the growth of the above semiconductor layer structure, and the diffused impurities invade the light absorption region. For this reason, the light absorption region is not completely depleted by a built-in voltage generated at the p-n junction. Therefore, a sufficient electric field is not generated in the MQW structure (light absorption layer) which controls the light extinction operation. Even if light absorption occurs once, electric charge generated by the light absorption remains, and no light absorption occurs any more. In other words, in the conventional optical modulator, light absorption hardly occurs. In contrast, when a reverse bias voltage is externally applied, the MQW structure is depleted, and light absorption increases as the reverse bias voltage to be applied increases, resulting in a large light extinction. FIG. 15 shows the voltage dependence of the extinction ratio, which explains this state. For the optimal operation, therefore, a complete depleted state must be set by externally applying a reverse bias voltage until the bias point shown in FIG. 15. For this purpose, an additional circuit such as a bias T is indispensable to the optimal operation. Assume that separate confinement heterostructure (SCH) layers that form an SCH exist between cladding layers and an MQW structure. In this case, even while a reverse bias voltage is externally applied, a weak electric field that barely causes depletion is applied to the upper and lower SCH layers sandwiching the light absorption layer. For this reason, in an optical modulator having an SCH structure, carriers generated by light absorption in the light absorption layer or the like can be extracted by only a weak electric field, and hence the traveling speed of carriers is low. In addition, the heterobarrier existing between the SCH layer and the cladding layer hinders carrier traveling, resulting in an increase in accumulation of carriers. When, therefore, high-intensity light is to be modulated, the amount of carriers generated by light absorption becomes larger than the amount of carriers extracted. The resultant accumulated carriers cause a nonuniform electric field or electric field shielding effect. This makes it impossible to realize high-speed operation. In order to suppress such carrier accumulation, there is available a technique of forming an SCH layer by using a semiconductor layer with a graded or abrupt composition and extracting carriers with maximum smoothness. This arrangement, however, extremely complicates crystal growth and requires a growth technique with extremely high precision, resulting in difficulty in manufacturing a device.
In a conventional device, upper and lower SCH layers are formed to a thickness equal to or more than the diffusion length of impurities so as to prevent p-type and n-type impurities in upper and lower cladding layers from reaching an MQW structure serving as a modulation layer with high concentrations even if they are diffused. This suppresses a deterioration in modulation function. Therefore, during modulation operation using a modulation voltage, the modulation voltage is applied throughout the upper and lower SCH layers and the MQW structure. However, only part of this modulation voltage is applied as an effective voltage component to the MQW structure which controls optical modulation characteristics. The voltage components applied to the upper and lower SCH layers are ineffective voltage components that do not directly contribute to modulation (high extinction) characteristics.
Although the device can be operated without externally applying a reverse bias voltage, a driving voltage having a very large amplitude equivalent to a bias is required. Furthermore, in this case, the electric field strength is not linear to voltage but is nonlinear thereto until the MQW structure is completely depleted. Reflecting the high extinction characteristics, therefore, the amplitude waveform of the transmitted light intensity is strongly distorted to exhibit extremely nonlinear characteristics.
As described above, the conventional semiconductor optical modulators are susceptible to the influences of operating temperatures. In addition, high reverse bias voltages must be externally applied to the modulators. For these reasons, various problems have arisen.